Some air purification systems incorporated into HVAC systems utilize a photocatalytic oxidation (PCO) process or other catalytic process. The PCO process uses ultraviolet light (UV) and airborne water vapor to activate a catalyst, typically titanium dioxide. The activated catalyst generates hydroxyl radicals. The hydroxyl radicals break down contaminants in the air, such as odors, chemicals, and other organic compounds into less harmful substances, such as carbon dioxide and water. Other filtering approaches may be used.